FIGS. 4A and 4B are illustrative views for showing aerodynamic characteristics of a rotor of a helicopter in forward flight. As shown in FIG. 4A, when a helicopter 1 with a rotor having a radius R which is rotating at a rotational angular velocity .OMEGA. advances at a ground velocity V, the airspeeds of an advancing blade where the rotational speed of the rotor is added to the ground velocity V and a retreating blade where the ground velocity V is subtracted from the rotational speed of the rotor are significantly different.
In particular, at a position of an azimuth angle .PSI. (angle measured counterclockwise from the rearward direction of the helicopter 1) of 90.degree. the airspeed of the advancing blade reaches a maximum and the airspeed of a tip of the advancing blade becomes .OMEGA..times.R+V. At a position of .PSI.=270.degree., on the other hand, the airspeed of the retreating blade reaches a minimum and the airspeed of a tip of the retreating blade becomes .OMEGA..times.R-V. The airspeed of an intermediate portion of the retreating blade takes a value obtained by proportional distribution of .OMEGA..times.R+V and .OMEGA..times.R-V. For example, when .OMEGA..times.R=795 km/h and V=278 km/h are assumed, the airspeed at a position of about 35% from the root end of the retreating blade becomes zero, as shown in FIG. 4A.
Since the airspeeds of the blades thus vary greatly while the blades make one revolution, various phenomena take place. On an advancing blade, drag coefficient Cd increases rapidly as the airspeeds approach the speed of sound. When the airspeeds are given in terms of Mach number M, drag divergence Mach number Mdd is defined as Mach number of a time when increment .DELTA. Cd of drag coefficient Cd divided by increment .DELTA. M of Mach number (.DELTA. Cd/.DELTA. M) becomes 0.1. Drag divergence Mach number Mdd depends on a blade aerofoil section, and it is said that the greater the value, the better the blade becomes because a higher airspeed of the blade can be achieved. It is common to set the airspeed of the tip of the advancing blade to around Mach 0.85.
On a retreating blade, on the other hand, since the airspeed thereof is significantly lowered, angle of attack .DELTA. of the retreating blade must be greater in order to produce a lift similar to that of the advancing blade. For this purpose, it is common to carry out pitch control wherein a pitch angle of the retreating blade is controlled in accordance to azimuth angle .PSI.. While the pitch angle of the blade is controlled as a sinewave function which has a minimum amplitude at .PSI.=90.degree. and a maximum amplitude at .PSI.=270.degree., angle of attack .alpha. of the blade in this case varies in the direction of span as shown in FIG. 4B due to flapping of the blade itself. For example, when the blade is at the position of .PSI.=90.degree., the angle of attack .alpha. becomes about 0.degree. at the root end and about 4.degree. at the tip end. When .PSI.=270.degree., the angle of attack .alpha. of the blade becomes about 0.degree. at the root end and about 16.degree. to 18.degree. at the tip end, thus exceeding the stall angle of attack.
Characteristics used for evaluating a retreating blade include maximum lift coefficient Clmax and stall angle of attack, the maximum lift coefficient Clmax defined as the maximum value of lift coefficient when the angle of attack .alpha. of a blade having a particular aerofoil section is gradually increased and reached the stall angle of attack. The blade is said to be better when the values of maximum lift coefficient Clmax and stall angle of attack are greater.
FIG. 5 is a graph showing an operating environment of helicopter rotor blades. The advancing blade at .PSI.=90.degree. has a Mach number near the drag divergence Mach number Mdd and a lift coefficient Cl of about zero. The blade at .PSI.=0.degree. and 180.degree. is in a hovering state which is independent of the ground velocity V, while Mach number M is about 0.6 and lift coefficient Cl is about 0.6. The retreating blade at .PSI.=270.degree. has a Mach number of 0.3 to 0.5 and a lift coefficient Cl near the maximum lift coefficient Clmax. As the blade makes a full revolution, Mach number and lift coefficient vary greatly by going around these states described above.
Hence a helicopter blade aerofoil is required 1) to have a large value of drag divergence Mach number Mdd, and 2) to have a large value of maximum lift coefficient Clmax, while a better flight performance of a helicopter is achieved when these values are greater.
There are known as prior arts related to helicopter blade aerofoils, Japanese Unexamined Patent Publication JP-A 50-102099(1975), JP-A 59-134096(1984), JP-A 63-64894(1988) and Japanese Examined Patent Publication JP-B2 62-34600(1987)
Recently such attempts have been proposed that helicopters commute regularly by using roof floors of buildings or open public spaces as heliports, and for which it is required to minimize the noise of helicopters in flight.
FIG. 6 is a graph showing frequency spectrums of noises generated by a helicopter. The noises of the helicopter are classified into several categories on the basis of origins of the noises, while harmonic components of the main rotor rotation frequency are distributed in a range from 10 to 100 Hz, harmonic components of the tail rotor rotation frequency are distributed in a range from 60 to 300 Hz, and broadband noise of the main rotor is distributed from 60 to 300 Hz. When the helicopter is flying at high speed, HSI (High-Speed Impulsive noise) is generated in a range from 60 to 300 Hz.